The invention relates to a vehicle powertrain and a method for upshifting a gear in the powertrain. The invention can be applied in heavy-duty vehicles, such as trucks, buses and construction equipment and in light vehicles such as cars. Although the invention will be described with respect to a truck, the invention is not restricted to this particular vehicle, but may also be used in other vehicles such as buses, construction equipment and cars.
In a vehicle powertrain, comprising an automated clutch arranged between a propulsion unit and an automated transmission, during a gear upshift when the clutch has been disengaged the rotational speed of the propulsion unit can be decreased to a rotational speed synchronized with a selected new higher gear with the help of an auxiliary brake, such as an engine compression release brake, exhaust brake or an electric generator drivingly connected to the propulsion unit. A control unit is arranged to control the auxiliary brake to decrease the rotational speed of the propulsion unit to a rotational speed synchronized with the speed of the new higher gear before the clutch can reengaged in order to drivingly connect the propulsion unit to the driven wheels via the new higher gear. When the control unit registers that the rotational speed of the propulsion unit has been synchronized with the rotational speed of the new gear, the control unit is arranged to reengage the clutch and the powertrain can propel the vehicle with the new higher gear engaged. Such a procedure is beneficial since the rotational speed synchronization with the auxiliary brake speeds up the gear shifting process, and the total tin e for upshift can be decreased.
Even though the braking with the propulsion unit auxiliary brake during a gear upshift works well it sometimes occurs that the propulsion unit auxiliary brake cannot be activated for some reason, which results in a much longer gear upshifting time, since the propulsion unit rotational speed has to decrease with the help of propulsion unit internal rotational friction.
It is desirable to provide a method and a system which eliminates the above problems.
It is desirable to provide a powertrain and a method for controlling an upshift in a powertrain equipped with an auxiliary brake and a transmission with clutch and engagable gears.
In this context, the term “auxiliary brake” is used to denote additional means for assisting braking of the vehicle. In vehicles, auxiliary brakes are used in addition to the service brakes and allow for prolonged brake application without requiring assistance from the service brakes. In this way, wear and unnecessary heating of the service brakes can be avoided. When the propulsion unit is an internal combustion engine such auxiliary brake can comprise an exhaust valve, for activating an exhaust brake, engine brake valves, for activating a compression release engine brake, which auxiliary brakes are dependent on engine speed and gear selection. Further, the auxiliary brake can be an electric generator in combination with a propulsion unit being either an internal combustion engine or an electric motor.
According to a first aspect of the invention, a vehicle powertrain is provided comprising a propulsion unit having a propulsion unit auxiliary brake, a transmission, driven wheels and a control unit arranged to control at least the transmission. The propulsion unit is drivingly connected to the driven wheels via a clutch and different engagable gear ratios in the transmission. The control unit is arranged to perform the following steps when upshifting:                automatically, disengage the clutch in order to disengage the propulsion unit from the transmission;        upshift the transmission to a new gear compared to current gear;        initiate a synchronization in order to decrease and synchronize rotational speed of the propulsion unit to rotational speed of the new gear to be engaged;        if it is determined by the control unit that the synchronization cannot be performed by decreasing rotational speed of the propulsion unit by the control unit controlling the auxiliary brake to brake, the rotational speed of the propulsion unit then,        automatically initiate a reengagement of the clutch in order to perform the synchronization.        
When the automatic transmission is about to be upshifted from a lower gear to a higher gear and the synchronization of the propulsion unit rotational speed with the auxiliary brake is not available, the control unit is arranged to use the clutch for achieving full synchronization of the rotational speed difference between the higher gear and the propulsion unit. In this way the control unit detects when the auxiliary brake is not available and automatically initiates rotational speed synchronization with the clutch instead of waiting for the rotational speed of the propulsion unit to decrease with help of internal friction of the propulsion unit. Another positive effect is that the rotational energy can instead be used to propel the vehicle forward instead of transform it to heat by braking the propulsion unit rotational speed down to the rotational speed of the new higher gear.
According to a first alternative example the vehicle powertrain further comprises a temperature sensor arranged to indicate temperature of the auxiliary brake and where the control unit is arranged to receive temperature information signals from the temperature sensor. The control unit is arranged to register that the synchronization cannot be performed by determining that braking performance from the auxiliary brake is unavailable when receiving temperature information signals from the temperature is below a first predetermined value and/or above a second predetermined value. Thus, according to one alternative if the auxiliary brake is too cold, that is, temperature is below a first predetermined value, then it is decided that the auxiliary brake is outside its operating temperature. According to another alternative if the auxiliary brake is too hot, that is, temperature is above a second predetermined value, then it is also decided that the auxiliary brake is outside its operating temperature. In a further alternative both a first and a second temperature value can be used, which means that a temperature interval is defined where the auxiliary brake is allowed to be operated.
As a further development of the previous example the temperature sensor is arranged to measure ambient temperature and/or temperature of the propulsion unit auxiliary brake and/or temperature of the propulsion unit.
According to a second alternative example the control unit is arranged to register that the synchronization cannot be performed by registering and determining that the auxiliary brake is faulty. Thus, the control unit is arranged to detect a faulty signal from the auxiliary brake. When the faulty signal is detected the control unit is arranged to use the clutch instead for the synchronization of the rotational speed difference.
According to a further alternative example the propulsion unit can be an internal combustion unit and the auxiliary brake can be an engine compression release brake.
In another alternative example of the invention the auxiliary brake is an electric generator and the propulsion unit is one of; an electric motor and an internal combustion unit. Thus, the powertrain can be a fully electric powertrain comprising an electric motor/generator drivingly connected to the driven wheels via a clutch and transmission with engagable gears, or the powertrain can be a hybrid electric powertrain comprising an internal combustion engine and at least an electric generator (preferably combined with an electric motor) where the electric generator can be used as an auxiliary brake and where the internal combustion engine is drivingly connected to the driven wheels via a clutch and engagable gears in the transmission.
According to a second aspect of the invention, a method is provided for gear upshifting in a vehicle powertrain, comprising a propulsion unit having a propulsion unit auxiliary brake, a transmission and driven wheels,
where the propulsion unit is drivingly connected to the driven wheels via the clutch and different engagable gear ratios in the transmission. The method comprising the following automatically performed steps when upshifting:                disengaging the clutch in order to disengage the propulsion unit from the transmission,        upshifting the transmission to a new gear compared to current gear;        performing a synchronization by decreasing and synchronizing rotational speed of the propulsion unit to rotational speed of the new gear to be engaged;        if determining that the synchronization cannot be performed by decreasing rotational speed of the propulsion unit by controlling the auxiliary brake to brake the rotational speed of the propulsion unit then,        initiating a reengagement of the clutch in order to perform the synchronization.        
The advantages are the same as with the corresponding device.
According to a first alternative example the method defines:
determining that the synchronization cannot be performed by measuring that a temperature is below a first predetermined value and/or above a second predetermined value where the auxiliary brake cannot operate.
According to a further alternative example the method defines:
determining that the synchronization cannot be performed by determining that the auxiliary brake is faulty.
According to another alternative example the method defines:
if determining that synchronization can only be performed partially by controlling the auxiliary brake, then automatically performing the synchronization by controlling the auxiliary brake and the reengagement of the clutch in parallel.
According to further aspects of the invention, a computer program, a computer readable medium and a control unit are provided, each being able to perform the steps of the method.
By the term “drivingly connected” typically means that a first component is connected to a second component in a manner allowing a transfer of a rotational movement and/or rotational torque from the first component to the second component. Therefore, the term encompasses a functional construction in which two components are connected such that the rotational speed of the first component corresponds to the rotational speed of the second component. However, the term also encompasses a functional construction in which there is a ratio between the rotational movement of the first component and the rotational movement of the second component, i.e., the rotational speed of the second component is proportional to the rotational speed of the first component.
Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing frond the scope of the present invention.